Studies on Range Expansion, Predation Pressure
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Species, Phylogeny and Evolution 1, 3 (30.9.2008): 129-140. Studies on range expansion, predation pressure and insola- tion in Albinaria on the island of Dia (Greece), focused on a recently introduced species (Gastropoda: Clausiliidae) Colette S. Mesher1 and Francisco W. Welter-Schultes2 1 C. S. Mesher, Marine Biology and Ecology Research Centre (MBERC), University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom 2 F. W. Welter-Schultes, Zoologisches Institut der Universität, Berliner Str. 28, 37073 Göttingen, Germany, [email protected] Abstract We examined the morphological properties of the introduced land snail Albinaria teres (Pulmonata: Clausiliidae) on the island of Día, north of Crete, Greece. Shells of four Albinaria species were sampled and measured from 68 localities evenly distributed over the island area. We tested possible correlations of shell parameters with insolation and predation pressure effected by Drilus beetles. We could not find any correlation of shell size, shape, number of whorls or teleoconch ribs of any species with insolation and predation pressure. The endemic A. torticollis, A. retusa and A. jaeckeli were more heavily predated by Drilus than the invader A. teres, which was morphologically less variable. The spatial distribution of shell size in A. teres exhibited a concentric pattern, with the largest snails being found at the periphery of the popula- tion's range, they decreased in size towards one site at the northwestern coast of the island, interpreted as the locality of introduction. Our observations suggest that A. teres is expanding its range and that during the process of range expansion, larger individuals have had a selective advantage (migration ability hypothesis). To determine the range expansion velocity C, we could not apply the C = (2 r D)0.5 formula, since we neither had experimental data to estimate the intrinsic growth rate, r, nor a previously determined spread rate to deduce the diffusion coefficient, D. We developed a model based on known data about annual displacements of individuals and the distribution of age classes in a natural population. Our simulation produced a mean rate of spread of 0.90-1.35 m yr-1, corresponding well with known land snail range expansion velocities. The mean distance between full population density and the first invaders of the new territory was 7.3 ± 1.7 m. If these values corresponded with reality, then the species was introduced at about 300 BC in the Greek period of Crete. Résumé Nous avons examiné les propriétés morphologiques du gastéropode terrestre Albinaria teres (Pulmonata: Clausiliidae) introduit à l’île de Día, au nord de la Crète, Grèce. Des coquilles de quatre espèces d’Albinaria ont été ramassées et mesurées de 68 stations distribuées sur toute l’île. Nous avons essayé de trouver des correlations des paramètres de la coquille avec l’insolation ainsi qu’avec la pression de prédation effectuées par des coléoptères du genre Drilus. Nous n’avons point trouvé une telle correlation ni avec la grandeur, la forme, le nombre de spires et les costilles de la coquille, dans aucune espèce. Les espèces endémiques A. torticollis, A. retusa et A. jaeckeli souffrirent plus fortement sous la prédation de Drilus que l’espèce invasive A. teres, qui était morphologiquement moins variable. Nous avons observé un cadre concentrique dans la distribution spatiale de la grandeur de la coquille d’A. teres, dont les coquilles les plus grandes ont été trouvés à la périphérie de l’aréal de distribution, diminuisant en grandeur vers une certaine localité au nord-ouest près de la côte, probablement l’endroit d’introduction, de sorte que les grandes individus ont eu un avantage sélective. Pour déterminer la vélocité d’expansion d’aréal de distribution C, nous n’avons pas pu utiliser la formule C = (2 r D)0.5, car nous n’avions ni des dates expérimentales pour estimer la vitesse interne de grandir r, ni une vitesse d’expansion déterminé avant pour déduir le coefficient de diffusion D. Nous avons développé un modèle basé sur des dates connus des movements annuaires des individus et la distribution des classes d’age dans une population naturelle. Notre simulation a produit une vitesse moyenne d’extention d’aréal de distribution de 0.90-1.35 m yr-1, correspondant bien aves des vélocités connus pour des gastéropodes terrestres. La distance moyenne entre une densité de population complète et les premiers invaseurs du nouveau territoire était à 7.3 ± 1.7 m. Si ces valeurs correspondent avec la réalité, l’espèce doit avoir être introduite environs vers 300 avant J.-C. dans l’époque grècque de la Crète. 129 Fig 1. Aspects of the landscape of Día. A. Boulder in square 53, at which 11 adult specimens of A. teres were found. Scenery seen from west; the approximate eastern range limit of the species is visible in the background (dotted line). B. Landscape of southern Día seen from the highest elevation at 268 m. Apart from local differences in the exposure, the island consists of a very uniform environment. Crete can be seen in the background. Introduction more size-dependent than of flying animals (Peters 1983). Predicting the rate of spread in animals bears two major Range expansion and invasions of new territories have problems. (i) For flying animals long-distance dispersal received considerable attention over the past decades is easier than for wingless animals. This increases the (Elton 1958, Lawton & Brown 1986, Baur & Bengtsson probability in flying animals that new territories will be 1987, Lubina & Levin 1988, Hengeveld 1989, Okubo et colonized not only by the populations at the periphery of al. 1989, Van den Bosch et al. 1990, 1992, Cushmann et the species' range. This will distort calculations based on al. 1993, Grosholz 1996, Lensink 1997). This paper is a pure neighborhood diffusion model (Hengeveld 1989). focused on whether larger land snails are more successful (ii) Some animal groups are sexual and have complicated invaders than smaller sized ones of the same species. Such social structures, for example terrestrial mammals. Also a study has never been undertaken in land snails. We have here, range growth cannot be described as a pure diffu- some data from insect studies, the conclusions of which sion-like mechanism, as for example in the case of Brow- can certainly not be directly applied for land snails, but nian motion of particles (Caughley 1970, Clarke 1971, we think they are interesting and worth to be cited in the Okubo et al. 1989, Van den Bosch et al. 1992). introduction. Being hermaphroditic animals without complicated A correlation between insect body size and success social structures, land snails are potentially more con- in migration or range expansion has been suggested in venient to study the nature of these processes. Their low several studies (Roff 1977, Harrison 1980, Cushmann et powers of dispersal additionally predestine land snails for al. 1993, Gutiérrez & Menéndez 1997). Cushmann et al. biogeographical studies. Land snails are good at coloni- (1993) introduced the expression "migration ability hypo- zing new territories either by natural means (Peake 1969, thesis" for this idea, but rejected this option to explain Vagvolgyi 1975, Valovirta 1979, 1984, Baur & Bengtsson the presence of larger-sized ants in more recently coloni- 1987) or by artificial dispersal by humans (Gittenberger zed regions of central and northern Europe. Size-depen- & Ripken 1987, Johnson 1988). In many regions of the dent dispersal ability of Drosophila flies was suggested eastern Mediterranean, the proportion of introduced snail by Roff (1977) who concluded that uncolonized habitats species exceeds 30 % (Mylonas 1984, Welter-Schultes should be colonized by larger individuals from the main 1998b, Welter-Schultes & Williams 1999). Snails were population. frequently carried accidentally on ships in the antiquity The velocity of range expansion has also been the sub- (Welter-Schultes 2008). ject of intensive research in both theoretical and experi- mental approaches (Lawton & Brown 1986, Lubina & Levin 1988, Okubo et al. 1989, Van den Bosch et al. 1990, The study object: Albinaria teres 1992, Matis et al. 1995, Grosholz 1996). As a general rule, larger animals can cover larger distances at higher velo- Albinaria is known for its fascinating variety of more cities, with velocities of strictly terrestrial animals being than 120 species and innumerable shell forms in Alba- 130 Fig 2. The island of Día north of Crete with 20 m contour lines, sampling grid and sampling sites. Akra Síderos in eastern Crete is the assumed origin of the artificially introduced A. teres population of Día. nia, Greece, Turkey, Cyprus and Lebanon (Böttger 1878, 1). The whole island is covered by shrubs of less than 1 Nordsieck 1977, 1999, Welter-Schultes 2000b). The high m in height, growing in a karstic terrain of pre-Neogene species number and the morphological variation does not (middle Jurassic to Eocene) limestone outcrops (IGME appear to be associated with a similar amount of ecolo- 1996). Día is protected by the Dasikí Ipiresía (Forestry gical differentiation, an observation that led Gittenberger Supervision) of the Cretan government and is mainly used (1991) to suspect non-adaptive radiation in the evolution to conserve one of the last populations of the Cretan wild of the genus. goat (Capra aegagrus cretica). The only environmental In Crete with its more than 20 species, this radiation is variable between sites seems to be the inclination of the believed to have occurred prior to the Tortonian (Douris hill slopes combined with their exposure (Fig. 1). The et al. 1998, Welter-Schultes 2000a). Some Albinaria highest elevation is at 268 m above sea level. populations have been artificially dispersed, for example Día is inhabited by four Albinaria species, three of from western Crete to the island of Lésvos (Nordsieck which were among the first land snails described from 1977, Bank 1988) or within Crete from one site to Greece (Olivier 1801, Wiese 1989, Schultes & Wiese another, over trajectories of 3 to 20 km (Welter-Schultes 1990, Welter-Schultes 1992).